Cortical GABAergic interneurons provide inhibition that is crucial for the computational power of the cerebral cortex. This class of inhibitory neuron exhibits a striking diversity of morphological, electrophysiological and connectivity features that together define various interneuron subtypes. How this diversity is accomplished and how various interneuron types further integrate into the cortex during development is not fully understood. We are interested in understanding the mechanisms underlying the maturation and establishment of specific interneuron types' connectivity patterns. Developmentally regulated genetic programs are known to regulate interneuron fate and maturation, although, they cannot account for the whole array of inhibitory neuron subtypes. Interestingly, recent work has shed light on the general requirement for neuronal activity in cortical interneuron migration, differentiation and possibly integration into circuits. However, the molecular connection between activity and interneuron integration into cortical networks remains to be elucidated. Activity-dependent alternative splicing of transmembrane proteins or ion channels has recently emerged as a potent post- transcriptional contributor to neuronal differentiation. Using RNA sequencing, we observed that similar functionally related transcripts are dynamically spliced in Parvalbumin (PV)-expressing basket and Somatostatin (SST)-expressing Martinotti cells during cortical network assembly. Therefore, we hypothesize that alternative splicing tailors the transcriptome to promote the maturation and integration of specific interneuron subtypes into developing cortical circuits. We recently identified the RNA binding protein, Fox-1 homolog (Rbfox1) to be developmentally enriched in the precursors that give rise to PV- and SST-expressing interneurons. Rbfox1 is splicing regulator that modulates the alternative splicing of an array of ion channels, neurotransmitter receptors and transmembrane proteins, affecting neuronal excitability and potentially synaptogenesis. Using state-of-the-art mouse genetics, genomics, neuroanatomical and electrophysiological approaches, we propose to investigate whether Rbfox1 regulates the establishment of specific synaptic connectivity between cortical interneurons and excitatory neurons and to identify Rbfox1-dependent splice variants directing the maturation and connectivity of specific interneuron subtypes (K99-phase). In addition, Rbfox1-dependent splicing regulation is modulated by membrane depolarization. Hence, we aim to unravel the contribution and the nature of neuronal activity in controlling alternative splicingin different interneuron subtypes (R00-phase). This application will be instrumental in uncovering fundamental mechanisms and molecules directing the maturation and the establishment of neuron subtype-specific connectivity. Understanding such mechanisms in vertebrates is an outstanding biological problem that warrants further investigations because synaptic defects are considered as a primary cause of intellectual disability and autism spectrum disorder.